Electronic implants can dramatically improve the quality of human life. Due to the development of nanoelectronics, there is now the possibility of integrating implants with more sophisticated physiological systems such as the visual system.
The visual system includes the retina, which receives light and converts it into electrical signals; the optic nerve, which communicates the signals from the retina to the brain; and the visual regions of the brain, which process the signals. Diseases such as macular degeneration deteriorate the retina while leaving the optic nerve healthy. Accordingly, to restore vision, researchers have proposed artificial retinal implants made using semiconductor chips that interface with healthy retinal neurons and communicate visual signals to the optic nerve. Retinal implants are promising because these semiconductor chips have not been rejected by the surrounding organic tissue of the eye and also because the brain and optical neurons have the capacity to learn how to interpret the signals originating from the retinal implants.
As shown in FIG. 1A, the retina has a layer of photoreceptors (rods and cones) 100 which detect incoming light 108. The layer of photoreceptors 100 is positioned behind a natural electrical circuit called the plexiform layer 102 which provides local processing of the electrical signal before passing the signal to the optic nerve (not shown). There are two general classes of retinal implants: epiretinal implants (FIG. 1B) and subretinal implants (FIG. 1C). As shown in FIG. 1B, an epiretinal implant 106 sits in front of the plexiform layer 102. In one epiretinal implant design, the implanted chip 106 wirelessly receives visual signals detected by a camera external to the eye. In contrast to the epiretinal implant, a subretinal implant 104, as shown in FIG. 1C, sits behind the plexiform layer 102, taking the place of deteriorated photoreceptors. One type of subretinal implant includes an array of photodiodes that convert light into electrical signals. For example, FIG. 2A is a close-up view showing a small portion of an array of 5000 photodiodes of a conventional subretinal implant. Each photodiode 200 in the array has a corresponding electrode 202 positioned at its center. When light is detected by photodiode 200, an electrical signal is communicated through the electrode 202 to the retinal neurons in the plexiform layer. FIG. 2B illustrates a retinal neuron having a central body called the soma 204 and branched projections called dendrites. The soma 204 is approximately 10 μm in width and the dendrites can extend the width of the neuron up to 100 μm. When dendrites connect with the electrode 202 of the implant, they form a neuro-electronic interface allowing the signals from the photodiode 200 to be communicated to the neuron and through the plexiform layer to the optical nerve and brain. The electrode 202 thus serves as a neuro-electronic interconnect of the implant.
The neuro-electronic interface between the electrode 202 and neuron is critical to the performance of the retinal implant. The design shown in FIG. 2A, however, has several potential problems. One significant problem is that electrical signals from the electrodes may damage the plexiform layer due to a capacitance overload at the interface or induce toxins due to polarization of electrolytic bio-fluid at the interface. One proposal for improving a neuro-electronic interface is to increase the roughness of the surface of the electrode. The increased surface area of the electrode is thought to reduce the probability that electrical signals will damage the neuro-electronic interface. This approach, however, does not solve other problems with the design.
Another potential problem is that many electrodes in the array may be unconnected or poorly connected to neurons in the plexiform layer. Although photodiodes can be fabricated with packing densities approaching those of the rods and cones in the human retina, a person with an artificial retina might still only be able to see a small fraction of the detail generated by the implant. Although increasing the size of each electrode 202 might be expected to improve the likelihood that it will connect with a neuron, this approach is not feasible for a retinal implant since it would obscure light from reaching the underlying photodiode 200 and defeat the fundamental purpose of the implant. The current design of FIG. 2A, in other words, suffers from a trade-off between capacity to sense light and capacity to communicate the resulting electrical signals to the neurons of the plexiform layer. There is thus a need for an improved method for forming a neuro-electronic interface which addresses the above limitations.
Moreover, electronic implants in other parts of the body also suffer from similar challenges related to insufficient number of neuro-electrical connections. Prosthetic hands, for example, do not have an adequate connection with the nerves to provide an adequate sense of touch. Similarly, the number of connections between neurons and electrodes implanted into mammalian brains is insufficiently large to accurately target neurons requiring electrical stimulation. The same problem is faced by implants designed to interface two parts of a biological system that have lost their natural connection. In each case, the functionality of the implant is limited by the performance of the neuro-electrical interface between the artificial and biological systems.
Accordingly, it is an object of the invention to address one or more of the above-mentioned disadvantages with current neuro-electronic interface techniques.